You need to pick the "Enough talk! Go to war!" option, and still have had the POSSIBILITY to not fight the Dredan/Matricks, which is based on your choice in scene03 and scene15 (if you were hostile before you can't get this achievement).

jack1974 wrote:You need to pick the "Enough talk! Go to war!" option, and still have had the POSSIBILITY to not fight the Dredan/Matricks, which is based on your choice in scene03 and scene15 (if you were hostile before you can't get this achievement).